Immunogens comprising capsular saccharide antigens conjugated to carrier proteins are well known in the art. Conjugation converts T-independent antigens into T-dependent antigens, thereby enhancing memory responses and allowing protective immunity to develop, and the prototype conjugate vaccine was for Haemophilus influenzae type b (Hib) [e.g. see chapter 14 of ref. 1]. Since the Hib vaccine, conjugated saccharide vaccines for protecting against Neisseria meningitidis (meningococcus) and against Streptococcus pneumoniae (pneumococcus) have been developed. Other organisms where conjugate vaccines are of interest are Streptococcus agalactiae (group B streptococcus) [2], Pseudomonas aeruginosa [3] and Staphylococcus aureus [4].
Rather than use full-length capsular saccharides, it is possible to select oligosaccharide fragments of desired size after a hydrolysis step [e.g. ref. 5], and it has been reported that conjugates made with intermediate chain-length oligosaccharides offer improved immunogenicity [e.g. refs. 6 & 7]. Of the three N. meningitidis serogroup C conjugated vaccines that have been approved for human use, MENJUGATE™ (N. meningitidis serogroup C oligosaccharide conjugate vaccine) [8] and MENINGITEC™ (N. meningitidis serogroup C oligosaccharide conjugate vaccine) are based on oligosaccharides, whereas NEISVAC-C™ (N. meningitidis serogroup C polysaccharide conjugate vaccine) uses full-length polysaccharide.
Where conjugates are included in a vaccine, quality control for manufacturing and release generally requires that they have a defined molecular size and/or molar mass, and also that these parameters are consistent between batches. Molecular size and molar mass can also be used for monitoring vaccine stability, as conjugates can aggregate over time, causing an increase in size and mass.
It is an object of the invention to provide new and improved methods for measuring molecular size, molar mass and related parameters for conjugated saccharide antigens, in particular for conjugates of meningococcal saccharides.